A dipole loudspeaker includes a rigid support containing an opening and a generally planar multi-layered flexible diaphragm mounted on the support and extending across the opening. The diaphragm is formed of a plurality of layers of thin flexible membrane material, each of the layers having a different...http://www.google.com.au/patents/US5148493?utm_source=gb-gplus-sharePatent US5148493 - Loudspeaker structure
